Conventional control systems for injecting fuel into internal combustion engines such as, for example, diesel engines typically operate in an open-loop control circuit. That is to say the injection time and the pulse width of the injection are extracted from permanently predefined characteristic diagrams or table memories which are stored in the engine's electronic control unit. Although such systems exhibit very fast control behavior, they are otherwise not very robust with respect to engine tolerances since the control strategy cannot be adapted in the case of faults. If, for example, the flow characteristics of an injector in a diesel engine changes as a result of wear phenomena, the pulse width used for the injector will no longer supply the engine with the desired quantity of fuel. This can result in higher emissions, higher fuel consumption, and increased noise and even damage to the engine. For these reasons, it is desirable to provide a feedback for combustion control in an internal combustion engine.
Combustion phasing can be utilized to correct injection control variables and composition of the cylinder charge. In this way, it is possible, for example, to compensate for service life drift of sensors and actuators such as an air mass flow rate sensor and a fuel injector.
However, such feedback control of an internal combustion engine depends on a feedback signal which can be used to characterize the combustion behavior. In this respect, measurements of the internal pressure of a cylinder have been proposed. A disadvantage of such measurements is the requirement of one sensor per cylinder, therefore, the high cost. Furthermore, the currently available sensors for the internal pressure of a cylinder exhibit a drift behavior and a relatively short service life. Direct access to the combustion chambers, which currently-available sensors require, is a problem.
The authors of SAE Paper 2006-01-0186 (M. Beasley, R. Cornwell, P. Fussey, R. King, A. Noble, T. Salamon, A. Truscott, and G. Landsmann: “Reducing Diesel Emissions Dispersion by Coordinated Combustion Feedback Control,” 2006) discuss in-cylinder pressure sensor signals for feedback control to reduce emissions in diesel engines. The authors recognize that in-cylinder pressure transducers are infeasible for production use and state: “low production cost, block mounted accelerometers were selected as a low-cost option for evaluation. At the time of writing, development of state estimation algorithms from engine block mounted accelerometers was continuing and details of the study will be published at a later date.”
It is well-known to use vibration sensors to detect knock in spark-ignition internal combustion engines. In that application, the major objective is to determine whether knock is occurring both reliably and robustly, i.e., over a wide range of operating conditions. Example references of such are U.S. Pat. Nos. 5,347,846 and 6,529,817. The inventors of the present invention have recognized a need to determine combustion phasing to use for feedback control. Such prior art knock detection does not provide the desired combustion phasing information
It is known in the art to use the spark plug as an ionization sensor to detect combustion phenomenon, examples are show in US Patent Applications 2004084034, 2004084021, 2004084036, and 2004088102. A small voltage is imposed across the spark plug gap, such voltage being much smaller than sparking voltage which causes a spark to jump the gap. The current flow from one electrode to the other is indicative of the number of ions at the spark plug gap, which is related to the state of combustion, pressure, and other factors.